On the Enterprise-D group of parasitic organisms attempted to consume the crews' cellular peptides (which would have caused them to collapse into pile of chemicals and thus die). However, they were discovered and purged from their bodies. (TNG: "Phantasms")

Some lifeforms, such as the Crystalline Entity, also consumed material, but converted it to energy directly rather than chemically breaking it down. (TNG: "Silicon Avatar") Some organisms fed on energy and either required a host, such as Ronin (TNG: "Sub Rosa") or fed on it directly, such as Farpoint Station.